This invention relates to a digital reflection densitometer system, and more particularly to a reflection densitometer which is both highly accurate and reliable in operation and extremely convenient to use.
Reflection densitometers are used in the graphic arts industry to perform a variety of functions. For example, color printing sheets are typically provided with a strip extending along one edge including bars of pure color, for example, black and shades of blue, red and yellow. When a particular sheet of this type has been approved for production, the color density of these color bars is determined with a reflection densitometer. Thereafter, during production runs, the color bars along the edges of corresponding printed sheets are checked with a reflection densitometer to assure that the approved color densities are being maintained.
Another use for reflection densitometers is in the area of photography. A reflection densitometer may be used to determine the density of the brightest or "highlight" areas and the darkest or "shadow" areas of the subject to be photographed. These values are then utilized in adjusting the exposure controls of the camera and thereby assuring a correct exposure.
Still another use of reflection densitometers relates to color film processing. Color film manufacturers provide test strips which if processed properly will bear color bars having known densitometer readings. These test strips may therefore be utilized to check the operating parameters of a processing system before the system is used to process exposed film.
It has been found that in order to facilitate the foregoing and similar operations, there are several characteristics which are highly desirable in a reflection densitometer. First, the instrument must be capable of providing highly accurate readings, and must be adapted for long term, essentially maintence free service. Second, the instrument should be adapted for one hand operation, since it is frequently necessary to take readings with one hand while simultaneously recording the reading with the other hand. Third, filter replacements, calibrations, and similar operations should be facilitated with a minimum of down time and inconvenience.
The present invention comprises a reflection densitometer which fulfills all of the foregoing requirements. In accordance with the broader aspects of the invention, all of the operating apparatus, substantially all of the controls, and the readout of a reflection densitometer are mounted in a single housing which is adapted for one hand manipulation over a surface. The operating apparatus includes a plurality of lamps for illuminating a predetermined area of the surface and a planar silicon sensor for producing an output indicative of the intensity of light reflected from the predetermined area of the surface. Switching apparatus is mounted on the housing and is adapted to position any one of a plurality of filters in the path of light passing to the planar silicon sensor, and to similtaneously regulate the operation of electronic circuitry coupled to the output of the planar silicon sensor. The electronic circuitry functions to drive a digital display mounted in the housing and adapted to provide a digital visual readout indicative of the output of the planar silicon sensor.
In accordance with more specific aspects of the invention, an optical system is mounted in the housing for directing light reflected from the predetermined area of the surface along an optical axis to the planar silicon sensor. The switching apparatus includes a shaft extending parallel to the optical axis, and a filter wheel secured to the shaft for rotation therewith to position any one of the filters within the optical system and in alignment with the optical axis thereof. The filter wheel includes a slot which receives the shaft to locate the filters relative to the axis of rotation of the shaft. The housing is provided with a slot which permits insertion, installation, and removal of the filter wheel and which provides access to a set screw adapted to secure the filter wheel to the shaft.
The electronic circuitry of the reflection densitometer comprises an operational amplifier coupled to the output of the planar silicon sensor, a logarithmic amplifier coupled to the output of the operational amplifier, and a digital volt meter coupled to the output of the logarithmic amplifier. The switching apparatus includes switches responsive to the rotational positioning of the shaft for varying the gain of the operational amplifier and for adjusting the sensitivity of the digital volt meter in accordance with the particular filter which is positioned within the optical system. The digital display device comprises a plurality of light emitting diodes which are driven by the digital volt meter to provide the digital visual readout.
In accordance with still other aspects of the invention, a foot is pivotally supported on the housing for engagement with the surface. The foot includes a target portion for designating the predetermined area of the surface. A spring is provided for normally pivotally separating the housing and the foot, however, the housing is adapted for manual pivotal movement into an operating position adjacent the foot against the action of the spring. The lamps are normally operated in a relatively low intensity state and are actuated to a relatively high intensity operating state whenever the housing is pivoted into the operating position.
Operating power for the lamps, the planar silicon sensor, the electronic circuitry, and the digital display device is provided by an electronic power supply. The power supply is mounted within a second housing which is connected to the first housing by means of an electrical cable of substantial length. This arrangement has been found to be highly satisfactory in that it substantially reduces both the weight and the bulk of the portion of the apparatus which is manipulated in order to make reflection densitometer readings, and yet does not sacrifice the convenience which is obtained by mounting all of the operating apparatus, substantially all of the controls, and the readout of the reflection densitometer within the same housing.